As demanded by car navigation systems, logistics, and the like, it has recently been desired to calculate running positions of vehicles. Known as a technique for calculating a running position of such a vehicle is one using a road-side device such as an optical beacon (e.g., Patent Literature 1). This technique takes out positional information (x, y) included in a beacon signal transmitted from an optical beacon and detects a moving direction and distance (Δx, Δy) by which a vehicle autonomously travels from a position where this positional information (x, y) is taken out. The autonomously traveled moving direction and distance (Δx, Δy) is added to the positional information (x, y), so as to calculate the current position (x+Δx, y+Δy) of the vehicle at that point of time.    Patent Literature 1: Japanese Patent Application Laid-Open No. 11-160085